Plastic bobbins are utilized in ballasts for fluorescent lights and the like. One form of such a ballast is disclosed in U.S. patent application Ser. No. 257,528, filed Oct. 14, 1988, entitled "An Improved Ballast", the disclosure of which is incorporated herein by reference. The ballast of the co-pending application is formed of plastic and is molded through the use of one or more die parts having suitable cavities formed therein so that a bobbin is created which includes an elongated rectangular body member having a plurality of flanges and stand-offs extending radially outwardly therefrom. At least some of the flanges are provided with small notches or the like which are adapted to receive the wires of the coil windings positioned on the rectangular body portion. The body portion is also provided with an elongated opening extending therethrouqh which is adapted to receive the central portion of a laminated core. In many bobbins, an elongated lamination compression ridge protrudes from the body portion into the central opening which is designed to exert pressure on the laminations of the central portion of the core to prevent vibration thereof. Since the central portion of the core is comprised of a plurality of laminations, the thickness of the same can vary from one core to another due to the permissible tolerances of the laminations. In those cases where the central portion of the core is slightly larger than the average, a portion of the compression ridge will be sheared when the core is inserted into the central opening of the bobbin. In those cases where the thickness of the central portion of the core is slightly less than the average, the lamination compression ridge will still exert pressure on the core to prevent vibration thereof.
During the molding of the plastic bobbin, the plastic material is normally injected through one or more sprue gates or injection openings provided at the opposite ends of the bobbin. Inasmuch as the parts of the bobbin are extremely small or thin at the core section of the bobbin and are located farthest away from the points of injection, high injection pressure must be utilized to ensure that the plastic material will be supplied to the internal cavities of the die parts. The fact that such high injection pressure is utilized where the bobbin parts are thicker with complex bobbin details results in objectionable "flashing" being created in the bobbin which must be subsequently trimmed or removed so as not to interfere with the fabrication of the ballast.
It is therefore a principal object of the invention to provide a method of molding a bobbin for a ballast.
A further object of the invention is to provide a method of molding a plastic bobbin for a ballast wherein the fluid plastic material is injected into opposite ends of the cavity for forming the lamination compression ridge so that the said cavity acts as an internal runner or passage for the fluid plastic material.
Still another object of the invention is to provide a method of molding a plastic bobbin which facilitates a higher material flow speed in the thin cross-section thereof that propagates to the vented thick section of the bobbin thereby reducing injection pressure.
A further object of the invention is to provide a method of molding a bobbin for ballast or the like which permits simplification of the mold design and improves the operational performance of the mold.
These and other objects will be apparent to those skilled in the art.